She pulled a prank; that’s not a joke, in and of itself, and it wasn’t a “dad joke”, which simply refers to particularly cheesy puns.
 
Humor is derived from the absurd. In the case of perfect puns, it is at the absurd implications of syntactically sound misinterpretations of words (or sentences), for example, or at the notion of someone being sufficiently daft to make such a misinterpretation. Absurdity is in the subversion of expectations.  
In that case, no expectations were set up for the reader - we have no reason to believe that such a fifth floor exists, nor that Starlight Glimmer would be particularly familiar with the number of floors in a given non-specified building in non-specified context.
 
Of course, expectations are subjective, and from an in-character perspective, one would find it absurd to be an elevator operator who is not familiar with how many floors are on the elevator they’re operating, and Starlight Glimmer would be proximate enough to that expectation to appreciate the humor of pranking an operator in such a manner. We, as readers of a fictional story where the expected behaviour of characters is already rather absurd by realistic standards, are too many layers of narrative and perspective removed from the elevator operator to realistically have a strong expectation as to their having internalized how many floors are on the elevator they’re operating.

@Background Pony #27B3  
https://www.youtube.com/watch?v=GfhyRjIcBbQ

@laofuzi  
Definitely not , at least that joke would be rock solid then.

Who makes your jokes for you, Maud Pie?!

@Background Pony #3AFC  
Did you miss how Spike picked Lily?

Starlight values those fingers too, if you know what I mean.

that is not a sexy meowr spike. pull those fingers away unless you want to lose them.

@Kamazeustra  
More that we didn’t get what was about it.

@FreezerBurned  
It was a dad joke. Unfunny. It wasn’t so much that nobody got the joke…more that they were trying to figure out why it was supposed to be so funny.

@FreezerBurned  
Maybe they got it but it didn’t make them laugh.

@Kamazeustra  

 
Somehow they didn’t get it. I don’t understand how you couldn’t get it.

Starlight feeling pissed even though her secret admiree is poking her butt is odd.

@Kamazeustra  

 
There was a joke?

Karma for all the teasing she puts him through.

bombed is putting it lightly

Starlight growling like a cat in that fourth panel

He needs those fingers to support your lazy plot. I think Starlight’s the only one who doesn’t have a job.

People didn’t get the joke?

Thank goodness, i thought i was just being thick.

he values his fingers much more than you do yours, Starlight

Now this is a real joke.

For once, Spike is the teaser and not the teasee.

I got the joke, Glimmy. It was just a dad joke.